1. Field of the Invention
The present invention relates to a piezoelectric ceramic material used in, for example, automotive equipment, actuators and sensors for various electrical equipment, etc.
2. Description of the Related Art
A piezoelectric material has the function of converting electrical energy into mechanical energy (or mechanical energy into electrical energy), and actuators, etc. using this function are known. The piezoelectric actuator, which generates a displacement corresponding to an applied voltage and has a fast response, is used as a drive unit for flow control valves, injectors or the like, and thus makes it possible to precisely control the flow rate or the injection amount, as the case may be. Also, an ultrasonic motor using ultrasonic oscillation of the piezoelectric element as a drive source is useful in small electrical equipment, and further finds a wide variety of practical applications such as an acceleration sensor for detecting the force due to the acceleration as an electrical signal and an ultrasonic sensor for detecting an object or measuring the distance by transmitting and receiving an ultrasonic wave.
A ceramic piezoelectric material generally has a composite perovskite structure expressed as A (bivalent) B (quadrivalent) O3. A known typical example is Pb(Ti, Zr)O3 or what is called PZT. There are also materials made by substituting a specific element for a part of the A site or the B site of PZT or adding various additives thereto. As an example, Japanese Examined Patent Publication No. 8-728 discloses a composition having the piezoelectric characteristic thereof improved by substituting La for a part of the Pb and adding not more than 1.0 weight % of MnO2. Various other attempts have been made to improve the piezoelectric characteristic by substituting Sr, Ba, Ca, etc. for the A site or Ni, Nb, etc. for the B site, or adding Cr2O3, etc.
These piezoelectric materials are normally classified roughly into two types. One is a material (soft material) of a high piezoelectric constant (high d), the features of which are that (1) the piezoelectric constant is high, (2) the dielectric loss is large, and (3) the Curie point is as low as about 100 to 150xc2x0 C. The other type is a material (hard material) with a high Curie point, which generally has the features that (1) the piezoelectric constant is small, (2) the dielectric loss is small, and (3) the Curie point is high.
The piezoelectric constant is a large factor affecting the displacement, and a soft material having a high dielectric constant is considered a promising material for attaining a large displacement. Due to the high dielectric loss, however, the electrical energy received cannot be effectively converted to the mechanical energy, resulting in a small displacement. Further, the lost energy is presented as thermal energy, and therefore the temperature of the material itself rises. Especially when the Curie point is approached, the polarization is lost and the displacement is liable to be lost.
The hard material, on the other hand, in spite of a small dielectric loss and a high electromechanical conversion efficiency, is small in piezoelectric constant d and therefore requires a high drive voltage to attain the desired displacement.
To attain a large displacement with a low drive voltage, a large piezoelectric constant d and a small dielectric loss are desired. As described above, however, the soft material has a large dielectric loss, while the hard material has a small piezoelectric constant. The dielectric loss of even the hard material, though small, is not zero. Therefore, some heat is generated, and the dielectric constant thereof, affecting the displacement, increases sharply at about the Curie point. To exhibit a stable piezoelectric characteristic in the operating temperature range, therefore, a material with a high Curie point is required.
As a material group expected to produce a high Curie point and a large displacement, the present inventors have studied the possibility of introducing a bivalent substitute atom such as Sr into the A site of the composite oxide expressed as Pb (Ti, Zr, Y, Nb) O3. In the case where the substitution amount of Sr is increased as required to obtain a high displacement, however, the Curie point is considerably decreased. Conversely, the suppression of the decrease of the Curie point prevents a sufficient displacement. Therefore, the development of a piezoelectric material which can provide a large displacement, without reducing the curie point considerably, is highly desired.
The primary object of the present invention is to provide a piezoelectric material having a high Curie point, which is superior in high temperature stability and which exhibits a large displacement at a low drive voltage.
In view of the above-mentioned situation, the present inventors have variously studied substitute atoms to be introduced into the A Site of the composite oxide of Pb (Ti, Zr, Y, Nb) O3 group, the optimum values of the ratio of substitution thereof and the ratio of each component element.
Thus, it has been found that a high displacement and a high curie point can be effectively attained by a piezoelectric material containing, as a main component, a composite oxide expressed by the general formula
[Pb{1xe2x88x92x1xe2x88x92x2xe2x88x92xcex1(x1xe2x88x92x2)}, A1x1, A2x2]
xe2x80x83{(Tiy, Zrz) (1xe2x88x92xcex2(x1xe2x88x92x2)xe2x88x92xcex3), (Y0.5, Nb0.5)xcex3}O3 xe2x80x83xe2x80x83(1)
where y+z=1, 1.15 less than z/y less than 1.30, 0 less than xcex3 less than 0.1, and A1, A2, x1, x2 satisfy specific conditions.